Various hydrometallurgical techniques have been developed for recovering metals such as zinc, nickel, copper, cobalt, lead, aluminum, titanium, and magnesium from sulfide and oxide ores, concentrates and intermediates. One such technique involves leaching the ore with a lixiviant that promotes dissolution of one or more metals into the leaching solution. Various compounds have been used individually as leaching agents in the lixiviant, for instance, sulfuric acid, hydrochloric acid, nitric acid, ferric chloride, ferric sulfate, cupric chloride and magnesium chloride. Recently there has been much work in the area of chloride-based leaching processes. All of these techniques inevitably involve the dissolution of iron, which then has to be removed from the system in order to facilitate the recovery of the value metals.
Iron is and has always been considered a major problem in hydrometallurgical processes such as those referred to above. In atmospheric processes, the iron is usually precipitated as an oxy-hydroxide, and in higher temperature autoclave processes, as an impure hematite. Often, small amounts of copper are added to act as a catalyst in the oxidation of ferrous to ferric. A more acceptable method of controlling iron is to form FeOOH, either β-FeOOH (akaganéite) or α-FeOOH (goethite) as described by D. Filippou and Y. Choi, “A Contribution to the Study of Iron Removal From Chloride Leach Solutions”, in Chloride Metallurgy 2002 Volume 2, (E. Peek and G. van Weert, Editors), Proceedings of the 32nd Annual CIM Hydrometallurgical Conference, CIM, Montreal (2002), p. 729. This approach is based to some extent on a controlled supersaturation precipitation technique, and is more efficient than, for example, the turboaeration process proposed by Great Central Mines in their chloride copper process, as described by R. Raudsepp and M. J. V. Beattie, “Iron Control in Chloride Systems”, in Iron Control in Hydrometallurgy (J. E. Dutrizac and A. J. Monhemius, Editors), Proceedings of 16th Annual CIM Hydrometallurgical Meeting, Toronto, October 1986, CIM Montreal (1996), p. 163.
Ferrous chloride solution, containing minor amounts of steel alloys such as manganese, vanadium and nickel, is the principal by-product of steel pickling lines (commonly referred to as waste pickle liquor, “WPL”). This solution is generally treated by a process called pyrohydrolysis, where the solution is injected into hot combustion gases at 700-900° C., causing oxidation of the ferrous iron to ferric and subsequent decomposition to recover hydrochloric acid and generate an iron oxide product for disposal or sale. The strength of the hydrochloric acid recovered from this process is limited to 18% because the off-gases have to be quenched in water, and using this method it is impossible to exceed the azeotropic concentration of hydrochloric acid in water, 20.4%.
The background to the present application has been largely covered in World Intellectual Property Organization International Publication Number WO2007/071020, Jun. 28, 2007 of Harris and White, which describes a process for the recovery of iron as hematite from a sulfide ore or concentrate. The process of Harris and White teaches a method for recovering iron as hematite from ferric chloride solutions containing a background chloride, preferably magnesium chloride, comprising heating the solution to 220-250° C. and adding water or steam to cause the precipitation of hematite and recovery of HCl.
U.S. Pat. No. 3,682,592 issued to Kovacs describes a process, the PORI Process, for recovering HCl gas and ferric oxide from waste hydrochloric acid steel mill pickle liquors (WPL). WPL typically contains water, 18 to 25% weight of ferrous chloride (FeCl2), less than 1% weight ferric chloride (FeCl3), small amounts of free hydrochloric acid and small amounts of organic inhibitors. The process of Kovacs includes two steps namely, a first oxidation step and a second thermal decomposition step. During the first oxidation step, the ferrous chloride in the WPL is oxidized using free oxygen to obtain ferric oxide and an aqueous solution containing ferric chloride. No hydrochloric acid is liberated at this stage. The first oxidation step is carried out under pressure (preferably, 100 p.s.i.g.) and at an elevated temperature (preferably, 150° C.), and therefore requires an autoclave.
During the second step, the resultant ferric chloride solution is thermally decomposed to obtain ferric oxide and HCl gas, which is recovered as hydrochloric acid. More specifically, the resultant solution is heated up to 175-180° C. at atmospheric pressure, and hydrolysis effected by the water in the fresh ferric chloride being added. The HCl is stripped off at a concentration of 30% with >99% recovery and good quality hematite is produced. While recovery of hydrochloric acid and hematite may be achieved using this process, its application tends to be limited to liquors containing only ferrous/ferric chlorides. When other chlorides are present in the solution, for instance and especially magnesium chloride as in the process of Harris and White, the activity of the chloride ions and protons tends to be too high to permit any reaction to take place simply by heating the solution to the temperature specified by Kovacs. Accordingly, this process tends not to be well adapted for use in leaching processes involving chlorides other than ferric chloride.
Applicant has found that the process of Harris and White will work in the laboratory in batch mode, but not in a continuous mode, because as the background chloride, e.g. magnesium chloride, calcium chloride, sodium chloride, aluminum chloride or base metal chlorides, concentration increases relative to that of iron, then the solution freezes, and is a solid at the temperatures indicated by Harris and White, and in some cases at the temperature indicated by Kovacs. This is true once the concentration of other chlorides reaches approximately 30% of the total in addition to ferric chloride. Accordingly, the processes of Harris and White and of Kovacs are impossible to operate if there are significant concentrations of other metal chlorides present in the solution.
SMS Siemag of Vienna, Austria, published a paper describing a process almost identical to that of Kovacs. The paper, Regeneração Hidrotérmica De Ácido Um Modo Econômico De Regenerar Líquidos De Decapagem E Produzir Óxidos Férricos De Alta Qualidade, published in Portuguese by Vogel, et al., follows the same procedures as Kovacs. More recently, a patent application describing the SMS Siemag process has been published by N. Takahashi et al., entitled Processing Method for Recovering Iron Oxide and Hydrochloric Acid, International Patent Application WO2009153321A1, Dec. 23, 2009. In the flowsheet published in the paper and patent application, the feed solution contains base metals such as manganese, and when this builds up, the liquid phase has to be discarded. This can be seen from FIG. 1 in the paper by Vogel et al. This is also similar to the observation noted by applicant in trying to reproduce the process of Harris and White.
In light of the foregoing, it would be advantageous to be able to both oxidise and hydrolyse ferrous iron in a single process, generating a high-strength stream of hydrochloric acid. Further, this hydrochloric acid may be used for recycle or re-use within the overall flowsheet, as well as a pure hematite product.